The elements in which last electron enters d-orbital of the
penultimate (inner to ultimate) shell, i.e last electron goes to (n-1)d orbital
are called d-block elements. D-block elements are also defined as those
elements whose two outermost shell are incomplete (i.e partially field)

The general valance shell electronic configuration of
d-block elements are also called transition elements. d-block elements is also called transition elements.

One of the most striking feature of the d-block elements is
that they exihibit variety of oxidation states in their compound with the
exception of a few elements most of the d-block elements show more than one
oxidation state (i.e variable oxidation state). The following two reasons have
been given to explain why d-block elements shows variable oxidation state.

(i)(n-1)d and ns orbitals in the atoms of d-block
elements have almost the identical energy and hence electron can be removed
from (n-1)d orbital as easily as they can be take part in different
hybridization situation.

(ii)After removing ns electron (s) the remainder is
called core or kernel. In case of most of d-block elements, the core is
unstable and hence can lase one or more elements resulting in the formation of
cation having different oxidation state i.e the removal of one or more electrons from the unstable core
gives several oxidation state to transition metals.